L-tyrosine-producing bacterium and a method for producing L-tyrosine

ABSTRACT

The present invention describes the production of L-tyrosine by culturing in a medium an  Escherichia  bacterium which has L-tyrosine-producing ability and which carries a mutant prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine, producing and accumulating L-tyrosine in the medium or in the bacterial cells, and collecting L-tyrosine from the medium or the bacterial cells.

FIELD OF THE INVENTION

The present invention relates to L-tyrosine-producing bacterium and a method for producing L-tyrosine by fermentation.

BRIEF DESCRIPTION OF THE RELATED ART

L-tyrosine is useful as a raw material or as a synthetic intermediate for pharmaceuticals. Conventional methods for producing L-tyrosine include extracting L-tyrosine from precipitates obtained during degradation of vegetable proteins, such as soybean proteins (JP07-10821 A), and producing L-tyrosine by fermentation using, for example, bacteria of the genus Brevibacterium (JP09-121872 A).

Bacteria of the genus Brevibacterium have an L-tyrosine biosynthetic pathway called the arogenate pathway, which produces arogenate from prephenate by prephenate aminotransferase, and then produces L-tyrosine from arogenate by arogenate dehydrogenase. Conventionally, L-tyrosine is produced by utilizing a bacterium of the genus Brevibacterium, since in this bacterium, the above-mentioned two enzymes are not inhibited when L-tyrosine is final product. However, growth of this bacterium is slow, and therefore productivity of L-tyrosine by this bacterium is low.

On the other hand, in the L-tyrosine biosynthetic pathway of Escherichia coli, prephenate is dehydrated by prephenate dehydrogenase (hereinafter, abbreviated to “PDH”) into 4-hydroxyphenylpyruvic acid, from which L-tyrosine is synthesized by aromatic amino acid aminotransferase. A problem in producing L-tyrosine by fermentation using Escherichia coli is that PDH, a first enzyme in this L-tyrosine-synthetic pathway, is subject to strong feedback inhibition by L-tyrosine, even with a low concentration of L-tyrosine. To efficiently produce L-tyrosine using Escherichia coli, the desensitization of this feedback inhibition by L-tyrosine is necessary first and foremost.

As for L-tyrosine production by Escherichia coli, it has been reported that bacteria resistant to p-fluoro-tyrosine excrete L-tyrosine (J. Bacteriol. 1958 September; 76(3): 328), and it was also reported that bacteria resistant to β-2-thienyl-alanine (J. Bacteriol. 1958 September; 76(3): 326), and bacteria resistant to P-aminophenylalanine (J. Bacteriol. 1969 March; 97(3): 1234) excrete L-tyrosine. However, mutations carried by those bacteria were not specified, or were found to be in a gene other than the PDH gene, such as 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DS) gene or the like. Therefore, neither L-tyrosine-producing bacterium carrying PDH which is desensitized to feedback inhibition, nor a mutation resulting in desensitization of feedback inhibition of PDH activity has been reported.

Although a strain of Bacillus bacterium in which feedback inhibition of PDH is desensitized was obtained from D-tyrosine-resistant strains (J. Biol. Chem. 1970 Aug. 10; 245(15): 3763), no mention has been made as to whether the desensitization of this inhibition actually results in enhanced production of L-tyrosine. Furthermore, a specific site of the mutation has not been examined. Furthermore, the procedure to obtain a D-tyrosine-resistant strain was not applicable to Escherichia coli, because Escherichia coli is inherently resistant to D-tyrosine due to the presence of the dadA (D-amino acid dehydrogenase A) gene (J. Bacteriol. 1994 March; 176(5): 1500).

In Escherichia coli, PDH is encoded by the tyrA gene and is identical to Chorismate mutase (CM) (SEQ ID NO: 1). It has been reported that PDH is inhibited by L-tyrosine at a concentration as low as 300 μM (Biochemistry. 1985 Feb. 26; 24(5): 1116). On the other hand, it was reported that the active sites of PDH are within the region of amino acids at positions 94 to 373, and that the region also includes sites involved in inhibition of its activity by L-tyrosine (Eur J. Biochem. 2003 February; 270(4): 757). However, it has not been elucidated which amino acid residue is involved in the inhibition.

3-fluoro-tyrosine is known as an inhibitor of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase and has been used to isolate a tyrosine repressor (tyrR)-deficient strain. Although 3-fluoro-tyrosine is also known to inhibit PDH activity, the use of 3-fluoro-tyrosine to obtain a PDH mutant has not been reported to date.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel L-tyrosine-producing bacterium belonging to the genus Escherichia and to provide a method for producing L-tyrosine using the Escherichia bacterium.

The inventors of the present invention extensively studied to arrive at the above-mentioned object, and as a result, it was found that an Escherichia bacterium harboring a mutant PDH which is desensitized to feedback inhibition by L-tyrosine can produce L-tyrosine efficiently and thereby, completed the present invention.

That is, the present invention provides the following:

It is an object of the present invention to provide an Escherichia bacterium which has L-tyrosine-producing ability comprising a mutant prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine.

It is an object of the present invention to provide the Escherichia bacterium as described above, wherein said mutant prephenate dehydrogenase comprises a protein in which one or more amino acids selected from the amino acids at positions 250 to 269 in a wild-type prephenate dehydrogenase is/are replaced by other amino acids.

It is an object of the present invention to provide the Escherichia bacterium as described above, wherein said mutant prephenate dehydrogenase comprises a protein in which an amino acid selected from the alanine at position 250, the glutamine at position 253, the alanine at position 254, the leucine at position 255, the histidine at position 257, the phenylalanine at position 258, the alanine at position 259, the threonine at position 260, the phenylalanine at position 261, the tyrosine at position 263, the leucine at position 265, the histidine at position 266, the leucine at position 267, the glutamic acid at position 269, and a combination thereof, is/are replaced by other amino acids.

It is an object of the present invention to provide the Escherichia bacterium as described above, wherein

-   the alanine at position 250 is replaced by phenylalanine, -   the glutamine at position 253 is replaced by leucine, -   the alanine at position 254 is replaced by serine, proline, or     glycine, -   the leucine at position 255 is replaced by glutamine, -   the histidine at position 257 is replaced by tyrosine, threonine,     serine, alanine, or leucine, -   the phenylalanine at position 258 is replaced by cysteine, alanine,     isoleucine, or valine, -   the alanine at position 259 is replaced by leucine, valine, or     isoleucine, -   the threonine at position 260 is replaced by glycine, alanine,     valine, cysteine, isoleucine, phenylalanine, asparagine, or serine, -   the phenylalanine at position 261 is replaced by methionine or     leucine, -   the tyrosine at position 263 is replaced by cysteine, glycine,     threonine, or methionine, -   the leucine at position 265 is replaced by lysine, isoleucine,     tyrosine, or alanine, -   the histidine at position 266 is replaced by tryptophan or leucine, -   the leucine at position 267 is replaced by tyrosine or histidine,     and -   the glutamic acid at position 269 is replaced by tyrosine,     phenylalanine, glycine, isoleucine, or leucine.

It is an object of the present invention to provide the Escherichia bacterium as described above, wherein said wild-type prephenate dehydrogenase is selected from the group consisting of:

-   -   (a) an amino acid sequence of SEQ ID NO: 2; and     -   (b) an amino acid sequence of SEQ ID NO: 2 including         substitution, deletion or addition of one or several amino acids         so long as said protein retains prephenate dehydrogenase         activity.

The Escherichia bacterium as described above, wherein said Escherichia bacterium is further modified so that expression of a gene encoding a prephenate dehydratase is reduced.

The Escherichia bacterium as described above, wherein said Escherichia bacterium is further modified so that expression of a gene encoding a tyrosine repressor is reduced.

The Escherichia bacterium as described above, wherein said Escherichia bacterium further comprises a 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase which is desensitized to inhibition by L-phenylalanine.

The Escherichia bacterium as described above, wherein said Escherichia bacterium is further modified so that expression of a gene encoding a shikimate kinase II is enhanced.

A method for producing L-tyrosine comprising culturing the Escherichia bacterium as described above in a medium, and collecting L-tyrosine from the medium or the bacterium.

A method for producing L-tyrosine or a derivative thereof comprising culturing an Escherichia bacterium in a medium, and collecting L-tyrosine or a derivative thereof from the medium or the bacterium, wherein said Escherichia bacterium is modified so that expression of the genes encoding a prephenate dehydratase and a tyrosine repressor is reduced, and expression of genes encoding a prephenate dehydrogenase and a shikimate kinase II is enhanced, and wherein said bacterium harbors a 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase which is desensitized to inhibition by L-phenylalanine.

A method of screening for a gene encoding a prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine comprising introducing a mutation into a gene encoding a wild-type prephenate dehydrogenase, introducing the mutated gene into a microorganism, selecting a microorganism having 3-fluorotyrosine resistance, and isolating a gene encoding a prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine from the selected microorganism.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the construction scheme of plasmid pSTVtyrA(mut).

FIG. 2 shows the PDH activity at each L-tyrosine concentration.

FIG. 3 shows the yields of L-tyrosine and L-phenylalanine by tyrA(mut)-introduced strain and a control strain. Each number corresponds to that in Table 1.

FIG. 4(A) shows positions of the mutations on the tyrA gene. FIG. 4(B) shows the construction scheme of plasmids pSTVtyrA(T260I) or pSTVtyrA(P100S).

FIG. 5 shows yields of L-tyrosine and L-phenylalanine by tyrA(T260I)-introduced strain and a control strain. Each number corresponds to that in Table 2.

FIG. 6 shows the construction scheme of plasmid pMGL.

FIG. 7 shows the yields of L-tyrosine and L-phenylalanine by tyrA(T260I)-introduced strain, pMGL-introduced strain, and a control strain. Each number corresponds to that in Table 5.

FIG. 8 shows of the PCR procedures for randomly introducing mutations into the amino acids of PDH at positions 250 to 270. Each of the primers (SEQ ID NOs: 15-35) contains a random codon (NNN) for mutating a target amino acid between the 5′-sequence and 3′-sequence each having 10-15 nucleotides and corresponding to a nucleotide sequence of the wild-type tyrA gene.

FIG. 9 shows the PDH activity of each mutant-introduced strain at each tyrosine concentration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail.

<1> The Escherichia Bacterium of the Present Invention

The Escherichia bacterium of the present invention has an L-tyrosine-producing ability and harbors a mutant PDH which is desensitized to feedback inhibition by L-tyrosine.

The Escherichia bacterium that can be used to obtain the Escherichia bacterium of the present invention may be any of those described in the work of Neidhardt, F. C. et. al. (Escherichia coli and Salmonella Typhimurium, American Society for Microbiology, p1208, Table 1), and for example, Escherichia coli. Examples of wild-type strains of Escherichia coli include K12 strain or a derivative thereof, Escherichia coli MG1655 strain (ATCC No. 47076), and W3100 strain (ATCC No. 27325). Those bacteria are available from, for example, the American Type Culture Collection (ATCC, Address: P.O. Box 1549, Manassas, Va. 20108, United States of America).

“The L-tyrosine-producing ability” refers to an ability to cause accumulation of a significant amount of L-tyrosine in a medium and/or an ability to increase the intracellular content of L-tyrosine, as compared to a wild-type strain or an unmutated strain. The Escherichia bacterium of the present invention may be a bacterium intrinsically having L-tyrosine-producing ability, or may be a bacterium modified to have L-tyrosine-producing ability. Furthermore, the Escherichia bacterium of the present invention may be a bacterium which gained L-tyrosine-producing ability by the introduction of a mutant PDH, or a mutant 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase which is desensitized to feedback inhibition by L-phenylalanine.

In the present invention, “PDH” refers to a protein which has an activity of catalyzing dehydration of prephenate to produce 4-hydroxyphenylpyruvic acid. Wild-type PDH refers to a PDH which is sensitive to feedback inhibition by L-tyrosine. An example of such a protein is a protein having the amino acid sequence of SEQ ID NO: 2. A wild-type PDH may be a protein having the amino acid sequence of SEQ ID NO: 2, which includes a substitution, deletion, or addition of one or several amino acids, so long as it retains the above-described activity. “Several” as used herein is specifically from 2 to 20, preferably from 2 to 10, more preferably from 2 to 5. The “substitution” is a mutation which deletes at least one amino acid and inserts another amino acid at the same position in SEQ ID NO: 2. Amino acid substitutions, though not particularly limited as long as the above activity is maintained, are preferably the following substitutions: substitution of ser or thr for ala; substitution of gln his or lys for arg; substitution of glu, gln, lys, his or asp for asn; substitution of asn, glu or gln for asp; substitution of ser or ala for cys; substitution of asn, glu, lys, his, asp or arg for gln; substitution of asn, gln, lys or asp for glu; substitution of pro for gly; substitution of asn, lys, gln, arg or tyr for his; substitution of leu, met, val or phe for ile; substitution of ile, met, val or phe for leu; substitution of asn, glu, gln, his or arg for lys; substitution of ile, leu, val or phe for met; substitution of trp, tyr, met ile or leu for phe; substitution of thr or ala for ser; substitution of ser or ala for thr; substitution of phe or tyr for trp; substitution of his, phe or trp for tyr; and substitution of met, ile or leu for val.

Moreover, a wild-type PDH may have a homology of 80% or more, preferably 90% or more, especially preferably 95% or more to the amino acid sequence of SEQ ID NO: 2. as long as it retains the above-described activity.

Furthermore, a wild-type PDH may be encoded by a DNA having a nucleotide sequence of SEQ ID NO: 1 or by a DNA hybridizable to a DNA having a nucleotide sequence of SEQ ID NO: 1 under stringent conditions and having the above-described activity. Examples of stringent conditions used herein include: a condition of one time, preferably two or three times, of washing at salt concentrations corresponding to 1×SSC, 0.1% SDS at 65° C., preferably 0.1×SSC, 0.1% SDS at 65° C.

On the other hand, “mutant PDH which is desensitized to feedback inhibition by L-tyrosine” refers to, for example, a PDH which exhibits the activity in the presence of L-tyrosine to the same extent as the activity in the absence of L-tyrosine. For example, such mutant PDH refers to a PDH which exhibits the activity in the presence of 100 μM L-tyrosine, not less than 50%, preferably not less than 80%, more preferably not less than 90% of the activity in the absence of L-tyrosine. Such mutant PDH can be obtained by introducing amino acid substitutions into wild-type PDHs and selecting a PDH which is not subject to feedback inhibition by L-tyrosine due to the amino acid substitution-introduced PDHs. Specifically, nucleotide substitutions causing amino acid substitutions are introduced into a DNA encoding PDH (e.g., SEQ ID NO: 1) by the use of, for example, site-directed mutagenesis using PCR or the like, and an Escherichia bacterium, preferably an Escherichia bacterium in which a wild-type PDH gene is disrupted is transformed with the mutated DNA. The PDH activity of the bacterium is then measured in the presence and absence of L-tyrosine, and a strain in which PDH is resistant to feedback inhibition by L-tyrosine is selected. PDH activity can be measured according to the method described in Biochemistry. 1990 Nov. 6; 29(44): 10245-54.

A gene encoding PDH which is desensitized to feedback inhibition by L-tyrosine can also be obtained by using 3-fluorotyrosine according to the following procedures. Namely, mutations are introduced into a gene encoding a wild-type PDH, the mutated gene is introduced into a microorganism, a 3-fluorotyrosine-resistant strain is selected, the PDH activity of the selected strain is measured in the presence and absence of L-tyrosine to thereby select a strain harboring a mutant PDH which is desensitized to feedback inhibition by L-tyrosine, and finally a gene encoding the mutant PDH is isolated from the selected strain.

Mutant PDH carried by the Escherichia bacterium of the present invention is not particularly limited as long as feedback inhibition by L-tyrosine therein is desensitized. Preferable examples of the mutant PDH include those having an amino acid sequence of wild-type PDH whereby one or more amino acids selected from the amino acids at positions 250 to 269 are replaced by the other amino acids.

More preferable examples of the amino acids to be replaced include one or more amino acids selected from the alanine at position 250, the glutamine at position 253, the alanine at position 254, the leucine at position 255, the histidine at position 257, the phenylalanine at position 258, the alanine at position 259, the threonine at position 260, the phenylalanine at position 261, the tyrosine at position 263, the leucine at position 265, the histidine at position 266, the leucine at position 267, and the glutamic acid at position 269.

Although the types of amino acids which are used to replace the above amino acids are not particularly limited as long as the amino acid substitution generates mutant PDH which is desensitized to feedback inhibition by L-lysine, the following substitutions are preferable: substitution of phenylalanine for alanine at position 250; substitution of leucine for glutamine at position 253; substitution of serine, proline or glycine for alanine at position 254; substitution of glutamine for leucine at position 255; substitution of tyrosine, threonine, serine, alanine or leucine for histidine at position 257; substitution of cysteine, alanine, isoleucine or valine for phenylalanine at position 258; substitution of leucine, valine or isoleucine for alanine at position 259; substitution of glycine, alanine, valine, cysteine, isoleucine, phenylalanine, asparagine, or serine for threonine at position 260; substitution of methionine or leucine for phenylalanine at position 261; substitution of cysteine, glycine, threonine or methionine for tyrosine at position 263; substitution of lysine, isoleucine, tyrosine or alanine for leucine at position 265; substitution of tryptophan or leucine for histidine at position 266; substitution of tyrosine or histidine for leucine at position 267; and substitution of tyrosine, phenylalanine, glycine, isoleucine or leucine for glutamic acid at position 269.

The numbering of the amino acid positions as used herein represent the positions in the amino acid sequence shown in SEQ ID NO: 2. These positions may be shifted ahead or backward by the aforementioned deletion, insertion, addition or inversion of one or several amino acids. For example, if one amino acid residue is inserted into the upstream position, the threonine originally located at the position 260 shifts to position 261. In the present invention, such threonine equivalent to “the threonine at position 260” is also referred to as the “threonine at position 260.” The same is true with regard to the other amino acid positions.

In order to introduce the above-mentioned mutant PDH into an Escherichia bacterium, the Escherichia bacterium may be transformed with, for example, a plasmid containing a DNA encoding the mutant PDH.

Although a DNA encoding a mutant PDH (also referred to as a mutant tyrA gene) is not particularly limited as long as it encodes a protein which has PDH activity and which is desensitized to feedback inhibition by L-threonine, examples of such DNA include a gene having a nucleotide sequence of SEQ ID NO: 1 or a nucleotide sequence able to hybridize with a polynucleotide having a nucleotide sequence of SEQ ID NO: 1 under stringent conditions, whereby a codon corresponding to one or more amino acids selected from the amino acids at positions 250 to 269 of SEQ ID NO: 2 is substituted by a codon corresponding to another amino acid.

Examples of vectors that can be used for introducing a mutant tyrA gene include a plasmid autonomously replicable in an Escherichia bacterium, and specifically include pUC19, pUC18, pHSG299, pHSG399, pHSG398, pACYC184, (pHSG and pACYC are available from Takara Bio Inc.), RSF1010, pBR322, and pMW219 (pMW is available from Nippon Gene Co., Ltd.).

The recombinant DNA containing the mutant tyrA gene prepared as described above can be introduced into an Escherichia bacterium by any known transformation method. Examples of transformation methods include treating recipient cells with calcium chloride so as to increase permeability of the DNA (Mandel, M. and Higa, A., J. Mol. Biol., 53, 159 (1970)), preparing competent cells from cells which are at the growth phase, followed by transformation with DNA (Duncan, C. H., Wilson, G. A. and Young, F. E., Gene, 1, 153 (1977)), and so forth.

The aforementioned mutant tyrA gene may also be integrated into a chromosomal DNA of the host bacterium. In order to integrate the gene into a chromosomal DNA of a bacterium, homologous recombination (Experiments in Molecular Genetics, Cold Spring Harbor Lab., 1972) may be carried out by targeting a sequence which exist in multiple copies on a chromosomal DNA. Repetitive DNA and inverted repeats at an end of a transposon can be used as a sequence which exists in multiple copies on a chromosomal DNA. Alternatively, as disclosed in EP0332488B, it is also possible to incorporate a target gene into a transposon, and allow it to be transferred so that multiple copies of the gene are integrated into the chromosomal DNA. Furthermore, a target gene may also be incorporated into a host chromosome by using Mu phage (EP0332488B).

Expression of the mutant tyrA gene may be controlled by the native promoter of the tyrA gene. Alternatively, the native promoter may be replaced with a stronger promoter such as the lac promoter, trp promoter, trc promoter, tac promoter, P_(R) promoter or P_(L) promoter of lambda phage, tet promoter, and amye promoter.

As methods for preparation of chromosomal DNA, preparation of a chromosomal DNA library, hybridization, PCR, preparation of plasmid DNA, digestion and ligation of DNA, transformation, design of oligonucleotides used as primers and so forth, ordinary methods known to those skilled in the art can be employed. These methods are described in Sambrook, J., Fritsch, E. F. and Maniatis, T., “Molecular Cloning A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press, (1989) and so forth.

The Escherichia bacterium of the present invention is preferably an Escherichia bacterium harboring the above-mentioned mutant PDH and further modified so that expression of a gene encoding prephenate dehydratase is reduced. An example of a gene encoding prephenate dehydratase (also referred to as a pheA gene) is a gene having a nucleotide sequence of SEQ ID NO: 3. The gene encoding prephenate dehydratase may also be a gene which is able to hybridize with a polynucleotide having a nucleotide sequence of SEQ ID NO: 3 under stringent conditions as long as it encodes a protein having prephenate dehydratase activity. Examples of the stringent conditions used herein include: washing one time, preferably two or three times, at salt concentrations corresponding to 1×SSC, 0.1% SDS at 65° C., preferably 0.1×SSC, 0.1% SDS at 65° C. Prephenate dehydratase activity can be measured according to a method described in Biochemica Biophysica Acta (BBA) 1965 (100) 76-88. It is preferred that the expression of a pheA gene is reduced to 10% or less as compared to that of unmodified strains, such as a wild-type strain. Expression of the pheA gene can be confirmed by measuring the amount of mRNA transcribed from the pheA gene in the bacterial cells by northern hybridization or RT-PCR. In the present invention, reduction of a pheA gene expression includes complete loss of expression. A modification to reduce the expression of a pheA gene can be carried out by gene disruption or alteration of an expression regulatory region such as a promoter. Specifically, such modification can be carried out by the method described below.

Gene disruption by homologous recombination has already been established, and examples thereof include a method using a linear DNA and a method using a plasmid containing a temperature-sensitive replication origin. An example of a plasmid containing a temperature-sensitive replication origin for Escherichia coli is pMAN031 (Yasueda, H. et al., Appl. Microbiol. Biotechnol., 36, 211 (1991)), pMAN997 (WO 99/03988), and pEL3 (K. A. Armstrong et. al., J. Mol. Biol. (1984) 175, 331-347).

A pheA gene on a host chromosome can be replaced with a deletion-type pheA gene in which part of its internal sequence is deleted, for example, as follows. That is, a recombinant DNA is prepared by inserting a temperature-sensitive replication origin, a deletion-type pheA gene, and a marker gene conferring resistance to a drug such as ampicillin or chloramphenicol into a vector, and an Escherichia bacterium is transformed with the recombinant DNA. Furthermore, the resulting transformant strain is cultured at a temperature at which the temperature-sensitive replication origin does not function, and then the transformant strain is cultured in a medium containing the drug to obtain a transformant strain in which the recombinant DNA is incorporated into the chromosomal DNA.

In a strain in which the recombinant DNA is incorporated into the chromosomal DNA as described above, the deletion-type pheA gene is recombined with the pheA gene originally present on the chromosome, and the two fusion genes of the chromosomal pheA gene and the deletion-type pheA gene are inserted into the chromosome so that the other portions of the recombinant DNA (vector segment, temperature-sensitive replication origin, and drug resistance marker) are present between the two fusion genes.

Then, in order to leave only the deletion-type pheA gene on the chromosomal DNA, one copy of the pheA gene is eliminated together with the vector segment (including the temperature-sensitive replication origin and the drug resistance marker) from the chromosomal DNA by recombination of two of the pheA genes. In this case, the normal pheA gene is left on the chromosomal DNA and the deletion-type pheA gene is excised from the chromosomal DNA, or to the contrary, the deletion-type pheA gene is left on the chromosomal DNA and the normal pheA gene is excised from the chromosome DNA. In both cases, the excised DNA may be harbored in the cell as a plasmid when the cell is cultured at a temperature at which the temperature-sensitive replication origin can function. Subsequently, if the cell is cultured at a temperature at which the temperature-sensitive replication origin cannot function, the pheA gene on the plasmid is eliminated along with the plasmid from the cell. Then, a strain in which pheA gene is disrupted can be obtained by selecting a strain in which the deletion-type pheA gene is left on the chromosome using PCR, Southern hybridization or the like.

By the way, a tyrosine repressor suppresses expression of the tyrA gene, or the like (J. Biol. Chem., 1986, vol. 261, p403-410). Thus, the bacterium of the present invention may be an Escherichia bacterium further modified to reduce the expression of a gene encoding a tyrosine repressor. Examples of a gene encoding a tyrosine repressor (also referred to as a tyrR gene) include a gene having a nucleotide sequence of SEQ ID NO: 5. A gene encoding a tyrosine repressor may also be a gene which is able to hybridize with a polynucleotide having a nucleotide sequence of SEQ ID NO: 5 under stringent conditions as long as it encodes a protein having tyrosine repressor activity. Examples of the stringent conditions used herein include: washing one time, preferably two or three times, at salt concentrations corresponding to 1×SSC, 0.1% SDS at 65° C., preferably 0.1×SSC, 0.1% SDS at 65° C. It is preferred that the expression of a tyrR gene is decreased to 10% or less as compared to that of unmodified strains such as a wild-type strain. In the present invention, reduction of a tyrR gene expression includes complete loss of the expression. A modification for reducing the expression of a tyrR gene can be carried out by gene disruption or alteration of an expression regulatory region such as a promoter. Specifically, this modification can be carried out in the same manner as in the disruption of the pheA gene as described above.

The bacterium of the present invention may be a bacterium which further comprises 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (also referred to as DS) which is desensitized to feedback inhibition by L-phenylalanine. Examples of DS which is desensitized to feedback inhibition by L-phenylalanine include a protein having an amino acid sequence of wild-type DS (e.g., SEQ ID NO: 8) encoded by aroG gene (e.g., SEQ ID NO: 7) whereby the proline at position 150 is replaced by leucine; a protein having an amino acid sequence of wild-type DS whereby alanine at position 202 is replaced by threonine; a protein having an amino acid sequence of wild-type DS whereby asparatic acid at position 146 is replaced by asparagine; a protein having an amino acid sequence of wild-type DS whereby methionine at position 147 is replaced by isoleucine and glutamic acid at position 332 is replaced by lysine; a protein having an amino acid sequence of wild-type DS whereby methionine at position 147 is replaced by isoleucine; and a protein having an amino acid sequence of wild-type DS whereby methionine at position 157 is replaced by isoleucine and alanine at position 219 is replaced by threonine (see JP 05-344881 A or JP 05-236947 A). A gene encoding either of these proteins is introduced into an Escherichia bacterium to obtain the Escherichia bacterium carrying DS which is desensitized to feedback inhibition by L-phenylalanine. Introduction of the gene can be carried out in the same way as the introduction of the mutant tyrA gene as described above.

The bacterium of the present invention may be further modified to increase the expression of a gene encoding shikimate kinase (aroL). An example of a gene encoding shikimate kinase is a gene having a nucleotide sequence of SEQ ID NO: 9. A gene encoding shikimate kinase may also be a gene which is able to hybridize with a polynucleotide having a nucleotide sequence of SEQ ID NO: 9 under stringent conditions as long as it encodes a protein having shikimate kinase activity. Examples of stringent conditions used herein include washing one time, preferably two or three times, at salt concentrations corresponding to 1×SSC, 0.1% SDS at 65° C., preferably 0.1×SSC, 0.1% SDS at 65° C. The gene is introduced into an Escherichia bacterium to obtain the Escherichia bacterium modified to increase the expression of the gene encoding shikimate kinase. Introduction of the gene can be carried out in the same way as the introduction of the mutant tyrA gene as described above.

As described above, the Escherichia bacterium of the present invention can be obtained by combining a tyrA mutation with the other modifications as described above. In the breeding of the Escherichia bacterium of the present invention, modification to mutate a tyrA gene and modification to reduce the expression of a gene encoding prephenate dehydratase and the like may be carried out in any order.

<2> Method for Producing L-Tyrosine

The method for producing L-tyrosine of the present invention includes culturing an Escherichia bacterium of the present invention in a medium, producing and causing accumulation of L-tyrosine in the medium or in the bacterial cells, and collecting L-tyrosine from the medium or the bacterium cells. The culture of the Escherichia bacterium of the present invention can be carried out in a similar manner as a conventional culturing method for an L-tyrosine-producing bacterium. Specifically, a typical culture medium contains a carbon source, a nitrogen source, and an inorganic ion, and if necessary, an organic micronutrient such as an amino acid or a vitamin. Examples of the carbon source include glucose, sucrose, lactose, and starch-hydrolyzed solution, whey and molasses containing the sugars. Examples of the nitrogen source include ammonia in the form of ammonia gas, ammonia water, or an ammonia salt. It is preferred that the culture is carried out under aerobic conditions with the pH and temperature of the medium appropriately controlled. A considerable amount of L-tyrosine is produced, and accumulates in the culture solution after the completion of culture. A known method can be used for collecting L-tyrosine from the culture.

The present invention also provides a method for producing L-tyrosine or a derivative thereof, which includes culturing in a medium an Escherichia bacterium modified so that expression of the genes which encode prephenate dehydratase and a tyrosine repressor is reduced, and expression of the genes which encode PDH and shikimate kinase II is enhanced and further carrying 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase which is desensitized to feedback inhibition by L-phenylalanine, producing and accumulating L-tyrosine or a derivative thereof in the medium or in the bacterial cells, and collecting L-tyrosine or a derivative thereof from the medium or the bacterial cells. The genes used for breeding the Escherichia bacterium in this method may be the same genes as those described above except that a wild-type PDH (tyrA) gene (e.g., a gene having a base sequence of SEQ ID NO: 1) is used instead of a mutant tyrA gene. The L-tyrosine derivative mentioned herein refers to a compound known to be produced from L-tyrosine by enhancing expression of one or more genes encoding tyrosine-metabolizing enzymes in E. coli, and examples thereof include melanin (European Patent No. 0547065 B1) and L-DOPA (JP 62-259589 A).

EXAMPLES

Hereinafter, the present invention will be described in detail with reference to the following non-limiting examples. The composition of the medium used in the Examples is as follows.

LB Medium:

-   Bactotryptone (manufactured by Difco) 10 g/L -   Yeast extract (manufactured by Difco) 5 g/L -   Sodium chloride 10 g/L -   pH 7.0 -   Twenty-minute steam sterilization at 120° C. was carried out.

LB Agar Medium:

-   LB medium IL -   Bacto Agar 15 g/L -   Twenty-minute steam sterilization at 120° C. was carried out.

Minimal Medium:

-   Glucose 0.2% -   Magnesium sulfate 1 mM -   Potassium dihydrogenphosphate 4.5 g/L -   Sodium citrate 0.5 g/L -   Ammonium sulfate 1 g/L -   Disodium phosphate 10.5 g/L -   Thiamine hydrochloride 5 mg/L -   Ten-minute steam sterilization at 115° C. was carried out.

Minimal Agar Medium:

-   Minimal medium 1 L -   Bacto Agar 15 g/L -   Ten-minute steam sterilization at 115° C. was carried out.

L-Tyrosine-Production Medium for Escherichia coli:

-   Glucose 40 g/L -   Ammonium sulfate 16 g/L -   Potassium dihydrogenphosphate 1.0 g/L -   Magnesium sulfate heptahydrate 1.0 g/L -   Iron(IV) sulfate heptahydrate 10 mg/L -   Manganese(IV) sulfate heptahydrate 8 mg/L -   Yeast extract 2.0 g/L -   Official calcium carbonate 30 g/L -   The components were dissolved in water, except for glucose and     magnesium sulfate heptahydrate, and the resulting solution was     adjusted with potassium hydroxide to pH 7.0 and sterilized at     115° C. for 10 minutes. Glucose solution and magnesium sulfate     heptahydrate solution were separately sterilized. -   L-phenylalanine and L-tyrosine were appropriately added. -   Chloramphenicol (25 mg/L) and ampicillin (100 mg/L) were added as     antibiotics.

EXAMPLE 1

<Acquisition of Mutant tyrA Gene Encoding A Mutant PDH which is Desensitized to Feedback Inhibition>

(1) Acquisition of a Mutant tyrA Gene Derived from Escherichia coli

Chromosomal DNA was extracted from an Escherichia coli K-12 W3110 strain according to a standard method. Two oligonucleotide primers having the nucleotide sequence shown in SEQ ID NO: 11 and 12 were synthesized, on the basis of the nucleotide sequence of wild-type tyrA gene described in J. Mol. Biol. 180(4), 1023 (1984). One of the primers has a nucleotide sequence corresponding to the region upstream of the ORF of the tyrA gene, and the other primer has a nucleotide sequence complementary to the region downstream of the ORF of the tyrA gene. Using the primers and the chromosomal DNA as a template, PCR was carried out to obtain an approximately 1-kbp DNA fragment containing the tyrA gene. As shown in FIG. 1, this fragment was digested with EcoRI and SalI and then ligated to pSTV28 (Takara Bio Inc.), and digested with EcoRI and SalI using a Ligation Kit Ver. 2 (Takara Bio Inc.). An Escherichia coli K-12 JM109 strain (Takara Bio Inc.) was transformed with the ligation product and the transformed strains were selected on a LB medium containing chloramphenicol. Introduction of the tyrA gene into the obtained chloramphenicol-resistant strain was confirmed by PCR. A plasmid was isolated from the strain containing the wild-type tyrA gene and named pSTVtyrA(W).

Next, a random mutation was introduced into a wild-type tyrA gene by PCR using the Gene Morph®D Random Mutagenesis Kit (STRATAGENE) and primers of SEQ ID NOs: 11 and 12, according to the manufacturer's instructions. The resulting mutated fragment was then ligated to pSTV28. The resulting plasmid containing the mutated tyrA gene fragment was used to transform Escherichia coli W3110 ΔtyrR,tyrA strain in which an endogenous tyrR gene and tyrA gene were disrupted. The transformed strains were selected on a minimal medium containing chloramphenicol and 0.1 mM 3-fluoro-tyrosine, and five 3-fluoro-tyrosine-resistant strains (W3110 ΔtyrR,tyrA/pSTVtyrA-1, -2, -5, -10, and -24) were tested for PDH activity. That is, the PDH activity of the selected strains was measured according to the procedure shown below.

The above-mentioned Escherichia coli W3110 ΔtyrR,tyrA strain can be obtained by curing the pMGAL1 plasmid (a plasmid which contains a feedback-desensitized aroG4 gene, a wild-type aroL gene, and a wild-type pheA gene) from the AJ12741 (FERM BP-4796) strain. The Escherichia coli AJ12741 strain was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology (currently, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Jun. 11, 1992 under the accession number of FERM P-13000. The original deposit was converted to an international deposit in accordance with the Budapest Treaty on Sep. 14, 1994, and given the accession number of FERM BP-4796.

(2) Measurement of PDH Activity

W3110 ΔtyrR,tyrA/pSTVtyrA-1, -2, -5, -10, and -24 strains were cultured at 37° C. for 15 hours in a LB medium, respectively and each culture solution was centrifuged to collect bacterial cells. Next, after the bacterial cells were washed twice with 50 mM Tris-HCl (pH 7.5) and suspended in 50 mM Tris-HCl (pH 7.5) containing 20% glycerol on ice, a crude enzyme solution was prepared by repeating 30 second-ultrasonication 80 times. Then, the PDH activity was measured according to the method described in Biochemistry. 1990 Nov. 6; 29(44): 10245-54. That is, the enzyme reaction was performed at 30° C. for 10 minutes in 50 mM Tris-HCl (pH 7.5) containing 0.25 mM prephenate, 1 mM EDTA, 1 mM DTT, and 2 mM NAD, and the generated NADH was measured at an absorption wavelength of 340 nm. A protein assay was carried out by the Bradford method. As shown in FIG. 2, it was found that one of the selected 5 strains harbors PDH which is desensitized to feedback inhibition by L-tyrosine. PDH activity was strongly inhibited in the presence of 100 μM L-tyrosine in a strain carrying a wild-type PDH as well as strains carrying pSTVtyrA-2, -5, -10, and -24, while the W3110 ΔtyrR,tyrA/pSTVtyrA-1 strain underwent almost no inhibition even in the presence of 800 μM L-tyrosine. Hereinafter, this tyrA gene inserted into pSTVtyrA-1 is referred to as tyrA(mut).

(3) Tyrosine Production of Escherichia coli Strain having a Mutant PDH which is Desensitized to Feedback Inhibition

W3110 ΔtyrR,tyrA strain was transformed with pSTVtyrA(W) or pSTVtyrA(mut) and transformants were spread over LB agar medium containing 25 mg/L of chloramphenicol. Thereafter, 1 cm² of the bacterial cells grown on the medium was scraped off and inoculated in 5 ml of the above-described L-tyrosine-production medium for Escherichia coli containing 25 mg/L of chloramphenicol, followed by culture with shaking at 37° C. for 24 hours. After the culture, 1 ml of the culture was taken and the glucose concentration therein was measured with a Biotech Analyzer (Sakura Seiki). That is, 1 ml of the culture was centrifuged at 12,000 rpm for 2 minutes and the supernatant was diluted with water to an appropriate dilution ratio, and the glucose concentration was measured.

The measurement of L-tyrosine and L-phenylalanine was carried out as follows. That is, KOH was added to the remaining culture solution to a final concentration of 0.3 M and then shaken at 37° C. for 1 hour to dissolve L-tyrosine, and part of the resulting solution was removed and diluted with water to an appropriate dilution ratio and the diluted solution was passed through Ultrafree-MC 0.45 μM Filter Unit (Millipore) to remove impurities, followed by analysis using HPLC system (HITACHI). Chromolith (MERCK) was used as an HPLC column and 5% acetonitrile/0.1% trifluoroacetic acid was used as buffer. The results are shown in Table 1 and FIG. 3. It was revealed that the accumulation of L-tyrosine was increased by 8 to 10 times by the mutation in tyrA gene, which desensitizes L-tyrosine-mediated feedback inhibition of PDH activity. TABLE 1 Remaining Phe Tyr sugar Yield Yield Sample g/l OD (×1/51) g/l (%) g/l (%) 1 W3110 tyrR, tyrA 0.0 0.356 2.17 5.8 2 W3110ΔtyrR, tyrA/pSTV28 0.0 0.415 2.35 6.3 n.d n.d 3 W3110ΔtyrR, tyrA/pSTV28 0.0 0.409 2.25 6.0 n.d n.d 4 W3110ΔtyrR, tyrA/pSTV-tyrA(wild) 0.1 0.550 0.05 0.1 0.06 0.1 5 W3110ΔtyrR, tyrA/pSTV-tyrA(wild) 0.1 0.615 0.08 0.2 n.d n.d 6 W3110ΔtyrR, tyrA/pSTV-tyrA(mut) 0.1 0.647 0.06 0.2 0.53 1.4 7 W3110ΔtyrR, tyrA/pSTV-tyrA(mut) 0.0 0.642 0.06 0.2 0.53 1.4 8 W3110ΔtyrR, tyrA/pSTV-tyrA(mut) 0.1 0.635 0.06 0.2 0.74 2.0 9 W3110ΔtyrR, tyrA/pSTV-tyrA(mut) 0.1 0.625 0.04 0.1 0.47 1.2 10 Blank 39.5 0.013 0.00 0.15 11 Blank 40.0 0.001 0.00 0.12

(4) Determination of Mutation Site of the Mutant tyrA Gene

The nucleotide sequence of the tyrA(mut) gene was determined according to a standard method. Mutation sites in the amino acid sequence of the mutant PDH encoded by the tyrA(mut) gene are shown in FIG. 4(A). In this mutant, proline at position 100 and threonine at position 260 were replaced by serine and isoleucine, respectively. In order to confirm if either of these mutations is important to the desensitization of the feedback inhibition of the mutant PDH, the following experiment was carried out (FIG. 4(B)). pSTVtyrA(mut) and pSTVtyrA(W) were digested with EcoRI, whose recognition site is present in the multi-cloning site upstream of the tyrA gene, and with CpoI, whose recognition site is present between the two mutation sites. The EcoRI-CpoI fragment which contains only the mutation at position 100 excised from pSTVtyrA(mut) was connected to the vector fragment of EcoRI-CpoI-digested pSTVtyrA(W), and thereby pSTVtyrA(P100S) which has a mutation only at position 100 was obtained. On the other hand, EcoRI-CpoI fragment containing no mutation excised from pSTVtyrA(W) was connected to the vector fragment of EcoRI-CpoI-digested pSTVtyrA(mut), and thereby pSTVtyrA(T2601) which has a mutation only at position 260 was obtained. Then, W3110 ΔtyrR,tyrA strain was transformed with each of those plasmids and the resulting strains were designated W3110 ΔtyrR,tyrA/pSTVtyrA(P100S) and W3110 ΔtyrR,tyrA/pSTVtyrA(T260I), respectively. The obtained strains were cultured and L-tyrosine production was evaluated. The results are shown in Table 2 and FIG. 5. It was revealed that the amino acid substitution at position 260 of PDH is important for desensitizing feedback inhibition. Accordingly, tyrA (T260I) was used in subsequent experiments. TABLE 2 Remaining Phe Tyr sugar Yield Yield Sample g/l OD (×1/51) g/l (%) g/l (%) 1 W3110ΔtyrR, tyrA/pSTV-tyrA(W) 0.0 0.495 0.20 0.60 0.19 0.57 2 W3110ΔtyrR, tyrA/pSTV-tyrA(W) 0.0 0.504 0.16 0.46 0.16 0.46 3 W3110ΔtyrR, tyrA/pSTV-tyrA(mut) 0.0 0.524 0.08 0.25 0.71 2.10 4 W3110ΔtyrR, tyrA/pSTV-tyrA(mut) 0.0 0.526 0.07 0.19 0.55 1.64 5 W3110ΔtyrR, tyrA/pSTV-tyrA(P100S)-1 0.0 0.485 0.07 0.22 0.13 0.39 6 W3110ΔtyrR, tyrA/pSTV-tyrA(P100S)-2 0.0 0.537 0.09 0.25 0.14 0.41 7 W3110ΔtyrR, tyrA/pSTV-tyrA(P100S)-3 0.0 0.588 0.03 0.08 0.08 0.23 8 W3110ΔtyrR, tyrA/pSTV-tyrA(P100S)-4 0.0 0.507 0.04 0.11 0.07 0.22 9 W3110ΔtyrR, tyrA/pSTV-tyrA(P100S)-5 0.0 0.510 0.08 0.23 0.16 0.48 10 W3110ΔtyrR, tyrA/pSTV-tyrA(T260I)-1 0.0 0.502 0.06 0.17 0.57 1.70 11 W3110ΔtyrR, tyrA/pSTV-tyrA(T260I)-2 0.0 0.578 0.08 0.23 0.53 1.58 12 W3110ΔtyrR, tyrA/pSTV-tyrA(T260I)-3 0.0 0.502 0.06 0.17 0.60 1.77 13 W3110ΔtyrR, tyrA/pSTV-tyrA(T260I)-4 0.0 0.466 0.10 0.28 0.59 1.74 14 W3110ΔtyrR, tyrA/pSTV-tyrA(T260I)-5 0.0 0.473 0.08 0.25 0.61 1.81 15 Blank 40.0 0.000 0.00 n.d 0.00 n.d 16 Blank 40.0 0.000 0.00 n.d 0.00 n.d

EXAMPLE 2

Acquisition of Another Mutant tyrA Gene Encoding a Mutant PDH which is Desensitized to Feedback Inhibition>

(1) Acquisition of Mutant tyrA Gene Derived from Escherichia coli

Primers for randomly inserting mutations into amino acid positions 250 to 270 of the amino acid sequence of PDH (SEQ ID NO: 2) were designed (SEQ ID NOs: 15 to 35).

Those primers and the primers of SEQ ID NOs: 11 and 12 were used in PCR to introduce the mutations. The procedure is as follows (FIG. 8).

First PCR: any of the primers SEQ ID NOs: 15 to 35 and a T2 primer (SEQ ID NO: 12) are used to amplify a portion of the tyrA gene downstream from the mutation introduction site, from the template of pSTV tyrA(W) using pyrobest DNA polymerase (TAKARA).

Second PCR: A second PCR is carried out in the same reaction solution as that of the 1st PCR. That is, as shown in FIG. 8, the second PCR amplification proceeds after the generation of 1^(st) PCR product. The PCR product amplified in the 1st PCR is denatured to sense and antisense strands, and the antisense strand functions as a primer to extend toward the upstream region of tyrA gene.

<1st/2nd PCR Cycle>

96° C., 2 min.,→>[94° C., 30 sec./60° C., 30 sec./72° C., 30 sec.]×20→[94° C., 1 min./37° C., 1 min./72° C., 30 sec.]×10→4° C.

Third PCR: The 3^(rd) PCR is performed to amplify a full length fragment of the tyrA gene which has mutations introduced using T1 (SEQ ID NO: 11) and T2 (SEQ ID NO: 12) primers, from the template of the mixture of the 2nd PCR products obtained by using each primer set.

<3rd PCR Cycle>

96° C., 2 min.,→[94° C., 30 sec./55° C., 30 sec./72° C., 1 min.] ×30→4° C.

The resulting PCR fragments which contain the mutant tyrA gene were ligated to the SmaI site of pSTV28 to obtain pSTV tyrA(mut). W3110ΔtyrR,tyrA strain was transformed with the resulting plasmids. The transformants were plated on a minimal medium containing chloramphenicol and 0.1 mM 3-fluoro-tyrosine and cultured at 24 hours at 37° C. Fifty strains were selected as candidate strains from the 3-fluoro-tyrosine-resistant strains.

(2) Tyrosine Production of the 3-Fluoro-Tyrosine-Resistant Strains

The selected transformants were spread on LB agar medium containing 25 mg/L of chloramphenicol. Thereafter, 1 cm² of the bacterial cells was scraped off and inoculated in 5 ml of the above-described L-tyrosine-production medium containing 25 mg/L of chloramphenicol, followed by culture with shaking at 37° C. for 24 hours. After culture, 1 ml of the culture solution was taken and the glucose concentration in the culture solution was measured. Glucose concentration and the amount of L-tyrosine which had been produced were measured according to the method as described above. The accumulation of L-tyrosine by each transformant is shown in Table 3. This table shows that accumulation of L-tyrosine was increased by 1.5 to 5.1 times by the introduction of the mutation into tyrA gene.

(3) Determination of Mutation Sites of the tyrA Gene Encoding PDH which is Desensitized to Feedback Inhibition

The nucleotide sequence of each mutant was determined according to a standard method. The codon and corresponding amino acid of the substitution site is shown in Table 3. TABLE 3 Mutation Accumulation Mutant or Position of amino Original Amino of L-tyrosine Wild type acid substitution Codon Amino acid Codon acid (%) Mutant 1 253 cag Q ttg L 1.5 Mutant 2 255 ctg L caa Q 1.04 Mutant 3 255 ctg L ctg G 1.48 Mutant 4 258 ttt F gta V 1.09 Mutant 5 260 act T att I 1.75 Mutant 6 254/269 gca/gaa A/E ggc/tac G/Y 2.24 Mutant 7 257/265 cac/ctg H/L tta/gca L/A 5.13 Wild type 0.35

(4) Measurement of PDH Activity

A strain containing mutant 7 (257/265), a strain containing mutant 6 (254/269), both shown in Table 3, and a strain containing the T2601 mutation shown in Table 2 were cultured at 37° C. for 15 hours in LB medium, and after the culture, the resulting culture was centrifuged to collect bacterial cells. Then, after the bacterial cells were washed twice with 50 mM Tris-HCl (pH 7.5) and suspended in 50 mM Tris-HCl (pH 7.5) containing 20% glycerol on ice, a crude enzyme solution was prepared by repeating 30 second-ultrasonication 80 times. PDH activity was measured according to the method described in Biochemistry. 1990 Nov. 6; 29(44): 10245-54. That is, a reaction was carried out at 30° C. for 10 minutes in 50 mM Tris-HCl (pH 7.5) containing 0.25 mM prephenate, 1 mM EDTA, 1 mM DTT, and 2 mM NAD⁺, and NADH was measured at an absorption wavelength of 340 nm. A protein assay was carried out by the Bradford method. As shown in FIG. 9, it was found that PDH activity was strongly inhibited in the presence of 100 μM L-tyrosine in a strain harboring a wild-type PDH, while strains harboring PDH containing the T2601 or 254/269 mutations underwent almost no inhibition even in the presence of 800 μM tyrosine. However, the PDH activity of the strain harboring the PDH with the 257/265 mutation could not be measured even though the L-tyrosine yield was high.

EXAMPLE 3

<Acquisition of Another Mutant tyrA Gene Encoding a Mutant PDH which is Desensitized to Feedback Inhibition>

Mutant tyrA genes were obtained similar to that described in Example 2 except that the 2^(nd) PCR products obtained by each primer set were not mixed but separately used as a template for 3^(rd) PCR. Three to ten clones of each transformant transformed with each of the 3^(rd) PCR products were analyzed as a candidate for mutant tyrA-containing strains. The culture and nucleotide sequence analysis of those strains were carried out in Example 2 and the result is shown in Table 4. It was found that the mutations which cause amino acid substitutions in the region from amino acids positions 250 to 269 of PDH resulted in improved L-tyrosine yield. TABLE 4 Yield of Position original mutation Tyr (%) (1) 250 gcg A ttt F 1.10 (2) 251 ttt F agt S 0.60 (3) 254 gca A tct S 0.50 (4) 254 gca A ccg P 0.98 (5) 254 gca A cct P 1.20 (6) 254 gca A gga G 0.60 (7) 257 cac H tac Y 5.50 (8) 257 cac H act T 3.00 (9) 257 cac H tca S 2.10 (10) 257 cac H gcg A 3.00 (11) 258 ttt F tgt C 0.58 (12) 258 ttt F gct A 0.50 (13) 258 ttt F ata I 0.74 (14) 259 gct A tta L 1.40 (15) 259 gct A gtt V 0.47 (16) 259 gct A ata I 1.50 (17) 260 act T ggt G 1.92 (18) 260 act T gga G 1.97 (19) 260 act T gct A 1.93 (20) 260 act T gtg V 1.95 (21) 260 act T gta V 1.92 (22) 260 act T tgt C 2.32 (23) 260 act T att I 2.97 (24) 260 act T ttc F 3.45 (25) 260 act T aat N 1.36 (26) 260 act T tct S 0.45 (27) 261 ttt F atg M 0.48 (28) 261 ttt F ctc L 0.64 (29) 263 tac Y tgt C 1.48 (30) 263 tac Y ggg G 2.55 (31) 263 tac Y acg T 2.00 (32) 263 tac Y atg M 1.10 (33) 265 ctg L aaa K 1.70 (34) 265 ctg L att I 0.53 (35) 265 ctg L tac Y 1.23 (36) 266 cac H tgg W 3.38 (37) 266 cac H tta L 1.16 (38) 267 ctg L tat Y 0.50 (39) 267 ctg L cat H 0.45 (40) 269 gaa E tac Y 1.63 (41) 269 gaa E gag E 0.44 (42) 269 gaa E ttt F 1.94 (43) 269 gaa E ggt G 2.14 (44) 269 gaa E ata I 1.31 (45) 269 gaa E ttg L 1.12 Wild 0.38

EXAMPLE 4

<Disruption of pheA Gene in the W3110 ΔtyrR,tyrA Strain>

The disruption of a prephenate dehydratase gene (pheA) was carried out by a method called “Red-driven integration”, which was first developed by Datsenko and Wanner [Proc. Natl. Acad. Sci. USA, 2000, vol. 97, No. 12, p6640-6645]. In this method, a DNA fragment containing a gene of interest and an antibiotic-resistant gene is used to obtain a gene-disrupted strain in one step. Based on the nucleotide sequence of a pheA gene described in J. Mol. Biol. 180(4), 1023 (1984) and a sequence of a NptII gene, which is a kanamycin-resistant gene from plasmid pCE1134 [Gene. 1982 October; 19(3): 327-36], a primer corresponding to a region in the proximity of the pheA gene and a primer complementary to a region in the proximity of the kanamycin-resistant gene were designed. The nucleotide sequences of these two primers are shown in SEQ ID NOs: 13 and 14. These primers were used to carry out PCR from the template of plasmid pCE1134.

The amplified PCR product was purified after separation by agarose gel electrophoresis, and used to transform W3110 ΔtyrR,tyrA strain containing a plasmid pKD46 having temperature-sensitive replicating ability (hereinafter referred to as W3110 ΔtyrR,tyrA/pKD46) by electroporation. The plasmid pKD46 [Proc. Natl. Acad. Sci. USA, 2000, vol. 97, No. 12, p6640-6645] contains a DNA fragment consisting of 2,154 nucleotides derived from λ phage (GenBank/EMBL accession No. J02459, Nos. 31088 to 33241) which includes genes of a λ Red system (λ, β, exo genes) which are controlled by an arabinose-inducible ParaB promoter. The temperature-sensitive plasmid pKD46 is required for the incorporation of the PCR product into the W3110ΔtyrR,tyrA strain.

Competent cells for electroporation were prepared as follows. W3110 ΔtyrR,tyrA/pKD46 strain was cultured overnight at 30° C. in a LB medium containing 100 mg/L of ampicillin, and then diluted 100 times with 5 mL of a LB medium containing ampicillin and L-arabinose (1 mM). The diluted cells were grown at 30° C. with aeration until OD600 reached approximately 0.6. The resulting cells were then washed three times with 1 mM ice-cold HEPES (pH 7.0) and used as competent cells for electroporation. Electroporation was carried out using 50 μL of the competent cells and approximately 100 ng of the PCR product. The cells after electroporation were supplemented with 1 mL of SOC medium [Molecular Cloning: A Laboratory Manual Vol. 2, Sambrook, J. et al., Cold Spring Harbor Laboratory Press (1989)] and cultured at 37° C. for 1 hour. The resulting culture was subsequently spread on a LB agar medium and cultured at 37° C. to select kanamycin-resistant strains. Next, for curing the pKD46 plasmid, the transformants were subcultured at 37° C. on a LB agar medium with kanamycin, and the resulting colonies were tested for ampicillin-resistance to select an ampicillin-sensitive strain which was created as a result of curing pKD46 from W3110 ΔtyrR,tyrA/pKD46 strain. A mutant strain which is kanamycin-resistant and in which the pheA gene is disrupted was confirmed by PCR. That is, it was confirmed that the length of a PCR product amplified from the DNA of a pheA gene-disrupted strain (W3110ΔtyrR,tyrA,pheA::NptII) was longer than that of the wild-type W3110AtyrR,tyrA,pheA strain, and that the kanamycin-resistant gene was inserted within the pheA gene. Thus, it was confirmed that the pheA gene was disrupted. The pheA-disrupted strain in which the kanamycin-resistant gene was inserted was designated W3110 ΔtyrR,tyrA,pheA::Km strain.

EXAMPLE 5

<Production of L-Tyrosine>

(1) Creation of Plasmid pMGL

pMGAL1 (a plasmid containing a mutant aroG4 gene resistant to feedback inhibition by L-phenylalanine, a wild-type aroL gene, and a wild-type pheA gene) which can be isolated from E. coli FERM BP-4796 strain was digested with HindIII to excise a pheA gene. The resulting plasmid, which was obtained by removing the pheA gene, was self-ligated to create a plasmid with pM119 containing only the mutant aroG4 gene and the wild-type aroL, which was designated as pMGL (FIG. 6).

(2) Creation of a Strain

The pSTVtyrA(T2601), pSTVtyrA(W) and pSTV28 as described in Example 1 were each introduced into the W3110 ΔtyrR,tyrA,pheA::Km strain obtained in Example 4 to create W3110 ΔtyrR,tyrA,pheA::Km/pSTVtyrA(T2601) strain, W3110 ΔtyrR,tyrA,pheA::Km/pSTVtyrA(W) strain, and W3110 ΔtyrR,tyrA,pheA::Km/pSTV28 strain, respectively. These strains were transformed with the above-obtained pMGL to create W3110 ΔtyrR,tyrA,pheA::Km/pMGL/pSTVtyrA(T2601) strain, W3110 ΔtyrR,tyrA,pheA::Km/pMGL/pSTVtyrA(W) strain, and W3110 ΔtyrR,tyrA,pheA::Km/pMGL/pSTV28 strain.

(3) Production of L-Tyrosine

Each of the transformed strains was cultured at 37° C. for 28 hours in the L-tyrosine-production medium. The results are shown in Table 5 and FIG. 7.

Analysis was carried out in the same way as in item (3) of Example 1. It was found that the introduction of the mutant tyrA gene resulted in improvement of the L-tyrosine yield, from 0.8-1.0 g/L to 3.3-3.5 g/L (approximately 5-fold improvement) in the W3110ΔtyrR,tyrA,pheA::Km strain, and from 3.9 g/L to 5.7-6.0 g/L (approximately 1.5-fold improvement) in the W3110ΔtyrR,tyrA,pheA::KmpMGL strain. Moreover, it was also found that production of L-phenylalanine as a byproduct does not occur as a result of the introduction of the mutant tyrA gene.

As shown in columns 11 and 12 of Table 5, the Escherichia coli strain in which the tyrR and pheA genes are disrupted and expression of the tyrA, aroL, and mutant aroG gene is enhanced, also gave a high yield of L-tyrosine. TABLE 5 Remaining Phe sugar OD Yield Tyr Sample (g/l) (×1/51) g/l (%) g/l 1 W3110ΔtyrR, tyrAΔpheA::Km 0.0 0.395 1.16 2.93 0.00 2 W3110ΔtyrR, tyrAΔpheA::Km 0.0 0.368 1.07 2.70 0.00 3 W3110ΔtyrR, tyrAΔpheA::Km/pSTV28 0.0 0.424 1.35 3.40 0.00 4 W3110ΔtyrR, tyrAΔpheA::Km/pSTV28 0.0 0.396 1.38 3.48 0.00 5 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.618 0.14 0.36 0.81 Km/pSTV-tyrA(wild) 6 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.611 0.11 0.27 1.11 Km/pSTV-tyrA(wild) 7 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.413 0.00 0.00 3.45 Km/pSTV-tyrA(T260I) 8 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.419 0.00 0.00 3.31 Km/pSTV-tyrA(T260I) 9 W3110ΔtyrR, tyrAΔpheA::Km/pMGL 2.0 0.376 1.26 3.35 0.00 10 W3110ΔtyrR, tyrAΔpheA::Km/pMGL 0.0 0.345 1.05 2.65 0.00 11 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.518 0.43 1.08 3.93 Km/pMGL/pSTV-tyrA(wild) 12 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.545 0.40 1.01 3.94 Km/pMGL/pSTV-tyrA(wild) 13 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.327 0.00 0.00 5.78 Km/pMGL/pSTV-tyrA(T260I) 14 W3110ΔtyrR, tyrAΔpheA:: 0.0 0.328 0.00 0.00 6.30 Km/pMGL/pSTV-tyrA(T260I) 15 Blank 41.8 0.000 0.00 n.d 0.00 16 Blank 41.8 0.000 0.00 n.d 0.00

While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents, including the foreign priority documents, JP2004-176797 and JP2005-112484, is incorporated by reference herein in its entirety. 

1. An Escherichia bacterium which has L-tyrosine-producing ability comprising a mutant prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine.
 2. The Escherichia bacterium according to claim 1, wherein said mutant prephenate dehydrogenase comprises a protein in which one or more amino acids selected from the amino acids at positions 250 to 269 in a wild-type prephenate dehydrogenase is/are replaced by other amino acids.
 3. The Escherichia bacterium according to claim 1, wherein said mutant prephenate dehydrogenase comprises a protein in which an amino acid selected from the alanine at position 250, the glutamine at position 253, the alanine at position 254, the leucine at position 255, the histidine at position 257, the phenylalanine at position 258, the alanine at position 259, the threonine at position 260, the phenylalanine at position 261, the tyrosine at position 263, the leucine at position 265, the histidine at position 266, the leucine at position 267, the glutamic acid at position 269, and combinations thereof, is/are replaced by other amino acids.
 4. The Escherichia bacterium according to claim 3, wherein the alanine at position 250 is replaced by phenylalanine, the glutamine at position 253 is replaced by leucine, the alanine at position 254 is replaced by serine, proline, or glycine, the leucine at position 255 is replaced by glutamine, the histidine at position 257 is replaced by tyrosine, threonine, serine, alanine, or leucine, the phenylalanine at position 258 is replaced by cysteine, alanine, isoleucine, or valine, the alanine at position 259 is replaced by leucine, valine, or isoleucine, the threonine at position 260 is replaced by glycine, alanine, valine, cysteine, isoleucine, phenylalanine, asparagine, or serine, the phenylalanine at position 261 is replaced by methionine or leucine, the tyrosine at position 263 is replaced by cysteine, glycine, threonine, or methionine, the leucine at position 265 is replaced by lysine, isoleucine, tyrosine, or alanine, the histidine at position 266 is replaced by tryptophan or leucine, the leucine at position 267 is replaced by tyrosine or histidine, and the glutamic acid at position 269 is replaced by tyrosine, phenylalanine, glycine, isoleucine, or leucine.
 5. The Escherichia bacterium according to claim 2, wherein said wild-type prephenate dehydrogenase is selected from the group consisting of: (a) an amino acid sequence of SEQ ID NO: 2; and (b) an amino acid sequence of SEQ ID NO: 2 which includes substitution, deletion, or addition of one or several amino acids so long as a protein having said amino acid sequence retains prephenate dehydrogenase activity.
 6. The Escherichia bacterium according to claim 1, which is further modified so that expression of a gene encoding prephenate dehydratase is reduced.
 7. The Escherichia bacterium according to claim 1, which is further modified so that expression of a gene encoding a tyrosine repressor is reduced.
 8. The Escherichia bacterium according to claim 1, which further comprises a 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase which is desensitized to inhibition by L-phenylalanine.
 9. The Escherichia bacterium according to claim 1, which is further modified so that expression of a gene encoding a shikimate kinase II is enhanced.
 10. A method for producing L-tyrosine comprising culturing the Escherichia bacterium according to claim 1 in a medium, and collecting L-tyrosine from the medium or the bacterium.
 11. A method for producing L-tyrosine or a derivative thereof comprising culturing an Escherichia bacterium in a medium, and collecting L-tyrosine or a derivative thereof from the medium or the bacterium, wherein said Escherichia bacterium is modified so that expression of the genes encoding a prephenate dehydratase and a tyrosine repressor is reduced and expression of genes encoding a prephenate dehydrogenase and a shikimate kinase II is enhanced, and wherein said bacterium comprises a 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase which is desensitized to inhibition by L-phenylalanine.
 12. A method of screening for a gene encoding a prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine comprising a) introducing a mutation into a gene encoding a wild-type prephenate dehydrogenase, b) introducing the mutated gene into a microorganism, c) selecting for a microorganism having 3-fluorotyrosine resistance, and d) isolating a gene encoding a prephenate dehydrogenase which is desensitized to feedback inhibition by L-tyrosine from the selected microorganism. 